1. Field of the Invention
The present invention relates to a plasma display apparatus and method of driving the same, and more particularly to, a plasma display apparatus and method of driving the same, which can improve a gray-level expression.
2. Description of the Background Art
Generally, a plasma display panel (PDP) excites and radiates a phosphorus material using an ultraviolet ray generated upon discharge of an inactive mixture gas such as He+Xe, Ne+Xe or He+Ne+Xe, to thereby display a picture inclusive of characters or graphics. Such a PDP is easy to be made into a thin-film and large-dimension type. Moreover, the PDP provides a very improved picture quality owing to a recent technical development. Especially, a three electrode AC surface discharge type PDP has wall charges accumulated in its surface upon discharge and protects its electrodes from the sputtering generated by the discharge, thus it has an advantage of low voltage drive and long life span.
Referring to FIG. 1, a discharge cell of a conventional three-electrode, AC surface-discharge PDP includes a scan electrode Y and a sustain electrode Z provided on an upper substrate 10, and an address electrode X provided on a lower substrate 18. Each of the scan electrode Y and the sustain electrode Z includes transparent electrodes 12Y and 12Z, and metal bus electrodes 13Y and 13Z having smaller line widths than the transparent electrodes 12Y and 12Z and provided at one edge of the transparent electrodes 12Y and 12Z.
The transparent electrodes 12Y and 12Z are usually formed from indium-tin-oxide (ITO) on the upper substrate 10. The metal bus electrodes 13Y and 13Z are usually formed from a metal such as chrome (Cr), etc. on the transparent electrodes 12Y and 12Z to thereby reduce a voltage drop caused by the transparent electrodes 12Y and 12Z having a high resistance.
On the upper substrate 10 provided, in parallel, with the scan electrode 30Y and the common sustain electrode 30Z, an upper dielectric layer 14 and a protective film 16 are disposed. Wall charges generated upon plasma discharge are accumulated onto the upper dielectric layer 14. The protective film 16 prevents a damage of the upper dielectric layer 14 caused by a sputtering during the plasma discharge and improves the emission efficiency of secondary electrons. This protective film 16 is usually made from magnesium oxide (MgO).
A lower dielectric layer 22 and barrier ribs 24 are formed on the lower substrate 18 provided with the address electrode X. The surfaces of the lower dielectric layer 22 and the barrier ribs 24 are coated with a phosphorous material 26. The address electrode X is formed in a direction crossing the scan electrode Y and the sustain electrode Z. The barrier rib 24 is formed in parallel to the address electrode X to thereby prevent an ultraviolet ray and a visible light generated by a discharge from being leaked to the adjacent discharge cells. The phosphorous material 26 is excited by an ultraviolet ray generated during the plasma discharge to generate any one of red, green and blue visible light rays. An inactive mixture gas is injected into a discharge space defined between the upper and lower substrate 10 and 18 and the barrier rib 24.
Such a PDP makes a time-divisional driving of one frame, which is divided into various sub-fields having a different emission frequency, so as to realize gray levels of a picture. Each sub-field is again divided into a reset period for initializing the entire field, an address period for selecting a scan line and selecting the cell from the selected scan line and a sustain period for expressing gray levels depending on the discharge frequency.
Herein, the reset period is again divided into a set-up interval supplied with a rising ramp waveform and a set-down interval supplied with a falling ramp waveform.
For instance, when it is intended to display a picture of 256 gray levels, a frame interval equal to 1/60 second (i.e. 16.67 ms) is divided into 8 sub-fields SF1 to SF8 as shown in FIG. 2. Each of the 8 sub-field SF1 to SF8 is divided into a reset period, an address period and a sustain period as mentioned above. Herein, the reset period and the address period of each sub-field are equal for each sub-field, whereas the sustain period and the number of sustain pulses assigned thereto are increased at a ratio of 2n (wherein n=0, 1, 2, 3, 4, 5, 6 and 7) at each sub-field.
FIG. 3 shows a driving waveform of the PDP applied to two sub-fields.
Referring to FIG. 3, the PDP is divided into a reset period for initializing, the full fields, an address period for selecting a cell, and a sustain period for sustaining a discharge of the selected cell for its driving.
In the reset period, a rising ramp waveform Ramp-up is simultaneously applied to all the scan electrodes Y in a set-up interval. This rising ramp waveform Ramp-up causes a weak discharge within cells at the full field to generate wall charges within the cells. In the set-down internal, after the rising ramp waveform Ramp-up was supplied, a falling ramp waveform Ramp-down falling from a positive voltage lower than a peak voltage of the rising ramp waveform Ramp-up is simultaneously applied to the scan electrodes Y. The falling ramp waveform Ramp-down causes a weak erasure discharge within the cells, to thereby erase spurious charges of wall charges and space charges generated by the set-up discharge and uniformly leave wall charges required for the address discharge within the cells of the full field.
In the address period, a negative scanning pulse scan is sequentially applied to the scan electrodes Y and, at the same time, a positive data pulse data is applied to the address electrodes X. A voltage difference between the scanning pulse scan and the data pulse data is added to a wall voltage generated in the reset period to thereby generate an address discharge within the cells supplied with the data pulse data. Wall charges are formed within the cells selected by the address discharge.
Meanwhile, a positive direct current voltage having a sustain voltage level Vs is applied to the sustain electrodes Z during the set-down interval and the address period.
In the sustain period, a sustain pulse sus is alternately applied to the scan electrodes Y and the sustain electrodes Z. Then, a wall voltage within the cell selected by the address discharge is added to the sustain pulse sus to thereby generate a sustain discharge taking a surface-discharge type between the scan electrodes Y and the sustain electrode Z whenever each sustain pulse sus is applied. Finally, after the sustain discharge was finished, an erasing ramp waveform erase having a small pulse width is applied to the sustain electrode Z to thereby erase wall charges left within the cells.
The thus-driven PDP expresses a gray level using a number of sustain pulses supplied during the sustain period. However, the expression of gray levels using the number of sustain pulses leads to a problem in that an expressible gray level is restricted. In more detail, the sustain pulses supplied during the sustain period generates a sustain discharge, and gray levels are expressed according to the number of sustain discharges. Here, the light generated by the sustain discharge is set to a constant amount, thereby being unable to express fine gray levels. For instance, in a conventional PDP, there was no method for display gray levels corresponding to half the light generated by a sustain discharge.